JP,B 7-103593 discloses a technique for improving fuel economy in a traveling working vehicle equipped with a hydraulic pump and a traveling device by changing the output characteristic of an engine and the torque of the hydraulic pump in a stepwise manner. The disclosed prior art will be described below.
The traveling working vehicle disclosed in JP,B 7-103593 includes an electronic controlled governor capable of changing the output characteristic of the engine in a stepwise manner, and an operating switch for outputting a mode selection signal. When an operator manipulates the operating switch and selects an M1 mode, the output characteristic of the engine is set to a general one that is the same as the conventional characteristic. When the operator selects other mode (any of M2 and M3 modes) than the M1 mode, the output characteristic of the engine is set to provide a smaller engine output torque than that in the M1 mode. Further, the traveling working vehicle includes two fixed displacement hydraulic pumps and a solenoid pilot cutoff valve for selectively connecting a delivery hydraulic line of one of those two hydraulic pumps to a drain circuit. When a transmission of the traveling device is shifted from a forward second speed (F2) to a forward first speed (F1) for excavation work in the state of the M2 mode being selected, an electric command is outputted to the solenoid pilot cutoff valve to unload the delivery hydraulic line of one of the two hydraulic pumps for a shift from 2-pump driving to 1-pump driving. Thus, when an operating mechanism hydraulic circuit is under high pressure (i.e., in the case of heavy-load work), a sufficient tractive force for traveling is ensured and the work rate is maintained. When the operating mechanism hydraulic circuit is under low pressure, a hydraulic load (pump absorption torque) is reduced from that in the case of 2-pump driving to distribute an engine output to the driving side for traveling at a higher ratio, whereby work performance can be ensured and fuel economy can be improved.
Japanese Patent No. 2968558 discloses such a technique that, when the sum of respective loads of a travel driving unit and an actuator is smaller than an engine output torque, a maximum absorption torque of a hydraulic pump is increased for distribution of the engine output torque to the operating mechanism side, and when the load sum is larger than the engine output torque, the maximum absorption torque of the hydraulic pump is reduced to ensure a large traveling torque for maintaining a large tractive force.
Patent Reference 2: Japanese Patent No. 2968558